revert MySQL to use DESCRIBE for has_table()

Restored the behavior of :meth:`.Inspector.has_table` to report on
temporary tables for MySQL / MariaDB. This is currently the behavior for
all other included dialects, but was removed for MySQL in 1.4 due to no
longer using the DESCRIBE command; there was no documented support for temp
tables being reported by the :meth:`.Inspector.has_table` method in this
version or on any previous version, so the previous behavior was undefined.

As SQLAlchemy 2.0 has added formal support for temp table status via
:meth:`.Inspector.has_table`, the MySQL /MariaDB dialect has been reverted
to use the "DESCRIBE" statement as it did in the SQLAlchemy 1.3 series and
previously, and test support is added to include MySQL / MariaDB for
this behavior.   The previous issues with ROLLBACK being emitted which
1.4 sought to improve upon don't apply in SQLAlchemy 2.0 due to
simplifications in how :class:`.Connection` handles transactions.

DESCRIBE is necessary as MariaDB in particular has no consistently
available public information schema of any kind in order to report on temp
tables other than DESCRIBE/SHOW COLUMNS, which rely on throwing an error
in order to report no results.

Fixes: #9058
Change-Id: Ic511bd5989ec17beb37b7cddd913732b626af0e6

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